Polyubiquitination and proteasomal turnover controls the anti-apoptotic activity of Bcl-B

Bcl-B: an "unknown" protein of the Bcl-2 family

Bcl-B is a poorly understood protein of the Bcl-2 family that is highly expressed in many healthy tissues and tumor types. Bcl-B is considered an antiapoptotic protein, but many reports have revealed its contradictory roles in different cancer types. In this mini-review, we elucidate the functions of Bcl-B in normal conditions and various pathologies, its regulation of programmed cell death, its oncogene/oncosuppressor activity in tumorigenesis, its impact on drug-acquired resistance, and possible approaches to inhibit Bcl-B.

N-terminal acetylation can stabilize proteins independent of their ubiquitination

The majority of proteins in mammalian cells are modified by covalent attachment of an acetyl-group to the N-terminus (Nt-acetylation). Paradoxically, Nt-acetylation has been suggested to inhibit as well as to promote substrate degradation. Contrasting these findings, proteome-wide stability measurements failed to detect any correlation between Nt-acetylation status and protein stability. Accordingly, by analysis of protein stability datasets, we found that predicted Nt-acetylation positively correlates with protein stability in case of GFP, but this correlation does not hold for the entire proteome. To further resolve this conundrum, we systematically changed the Nt-acetylation and ubiquitination status of model substrates and assessed their stability. For wild-type Bcl-B, which is heavily modified by proteasome-targeting lysine ubiquitination, Nt-acetylation did not correlate with protein stability. For a lysine-less Bcl-B mutant, however, Nt-acetylation correlated with increased protein stability, likely due to prohibition of ubiquitin conjugation to the acetylated N-terminus. In case of GFP, Nt-acetylation correlated with increased protein stability, as predicted, but our data suggest that Nt-acetylation does not affect GFP ubiquitination. Similarly, in case of the naturally lysine-less protein p16, Nt-acetylation correlated with protein stability, regardless of ubiquitination on its N-terminus or on an introduced lysine residue. A direct effect of Nt-acetylation on p16 stability was supported by studies in NatB-deficient cells. Together, our studies argue that Nt-acetylation can stabilize proteins in human cells in a substrate-specific manner, by competition with N-terminal ubiquitination, but also by other mechanisms that are independent of protein ubiquitination status.

Bcl-2-like protein-10 increases aggressive features of melanoma cells

Aim:

B-cell lymphoma-2 (Bcl-2)-like protein-10 (Bcl2L10) is the less studied member of Bcl-2 family proteins, with the controversial role in different cancer histotypes. Very recently, Bcl2L10 expression in melanoma tumor specimens and its role in melanoma response to therapy have been demonstrated. Here, the involvement of Bcl2L10 on the in vitro and in vivo properties associated with melanoma aggressive features has been investigated.

Methods:

Endogenous Bcl2L10 protein expression was detected by western blotting analysis in a panel of patient-derived and commercially available human melanoma cells. In vitro assays to evaluate clonogenicity, cell proliferation, cell migration, cell invasion, and in vitro capillary-like structure formation [vasculogenic mimicry (VM)] have been performed by using human melanoma cells stably overexpressing Bcl2L10 or transiently transfected for loss/gain function of Bcl2L10, grown under two- or three-dimensional (3D) conditions Xenograft melanoma model was employed to evaluate in vivo tumor growth and angiogenesis.

Results:

Results demonstrated that Bcl2L10 acts as an inducer of in vitro cell migration, invasion, and VM, while in vitro cell proliferation, in vivo tumor growth, as well as colony formation properties were not affected. Dissecting different signaling pathways, it was found that Bcl2L10 positively affects the phosphorylation of extracellular-signal-regulated kinase (ERK) and the expression of markers of cell invasion, such as urokinase plasminogen activator receptor (uPAR) and matrix metalloproteinases (MMPs). Of note, Bcl2L10-dependent in vitro migration, invasion, and VM are linked to uPAR. Bcl2L10 also negatively regulates the intracellular calcium level. Finally, reduced invasion capability in 3D spheroid invasion assay of melanoma cells transiently overexpressing Bcl2L10 was observed after treatment with inhibitors of MMPs and uPAR.

Conclusions:

Overall, data reported in this paper provide evidence supporting a positive role of Bcl2L10 in melanoma aggressive features.

Ubiquilin Networking in Cancers

Ubiquilins or UBQLNs, members of the ubiquitin-like and ubiquitin-associated domain (UBL-UBA) protein family, serve as adaptors to coordinate the degradation of specific substrates via both proteasome and autophagy pathways. The UBQLN substrates reveal great diversity and impact a wide range of cellular functions. For decades, researchers have been attempting to uncover a puzzle and understand the role of UBQLNs in human cancers, particularly in the modulation of oncogene's stability and nucleotide excision repair. In this review, we summarize the UBQLNs' genetic variants that are associated with the most common cancers and also discuss their reliability as a prognostic marker. Moreover, we provide an overview of the UBQLNs networks that are relevant to cancers in different ways, including cell cycle, apoptosis, epithelial-mesenchymal transition, DNA repairs and miRNAs. Finally, we include a future prospective on novel ubiquilin-based cancer therapies.

Chaperone-Mediated Autophagy and Its Emerging Role in Hematological Malignancies

Chaperone-mediated autophagy (CMA) ensures the selective degradation of cellular proteins endowed with a KFERQ-like motif by lysosomes. It is estimated that 30% of all cellular proteins can be directed to the lysosome for CMA degradation, but only a few substrates have been formally identified so far. Mechanistically, the KFERQ-like motifs present in substrate proteins are recognized by the molecular chaperone Hsc70c (Heat shock cognate 71 kDa protein cytosolic), also known as HSPA8, and directed to LAMP2A, which acts as the CMA receptor at the lysosomal surface. Following linearization, the protein substrate is next transported to the lumen of the lysosomes, where it is degraded by resident proteases, mainly cathepsins and eventually recycled to sustain cellular homeostasis. CMA is induced by different stress conditions, including energy deprivation that also activates macro-autophagy (MA), that may make it difficult to decipher the relative impact of both pathways on cellular homeostasis. Besides common inducing triggers, CMA and MA might be induced as compensatory mechanisms when either mechanism is altered, as it is the often the case in different pathological settings. Therefore, CMA activation can compensate for alterations of MA and vice versa. In this context, these compensatory mechanisms, when occurring, may be targeted for therapeutic purposes. Both processes have received particular attention from scientists and clinicians, since modulation of MA and CMA may have a profound impact on cellular proteostasis, metabolism, death, differentiation, and survival and, as such, could be targeted for therapeutic intervention in degenerative and immune diseases, as well as in cancer, including hematopoietic malignancies. The role of MA in cancer initiation and progression is now well established, but whether and how CMA is involved in tumorigenesis has been only sparsely explored. In the present review, we encompass the description of the mechanisms involved in CMA, its function in the physiology and pathogenesis of hematopoietic cells, its emerging role in cancer initiation and development, and, finally, the potential therapeutic opportunity to target CMA or CMA-mediated compensatory mechanisms in hematological malignancies.

Cellular demolition: Proteins as molecular players of programmed cell death

Apoptosis, a well-characterized and regulated cell death programme in eukaryotes plays a fundamental role in developing or later-life periods to dispose of unwanted cells to maintain typical tissue architecture, homeostasis in a spatiotemporal manner. This silent cellular death occurs without affecting any neighboring cells/tissue and avoids triggering of immunological response. Furthermore, diminished forms of apoptosis result in cancer and autoimmune diseases, whereas unregulated apoptosis may also lead to the development of a myriad of neurodegenerative diseases. Unraveling the mechanistic events in depth will provide new insights into understanding physiological control of apoptosis, pathological consequences of abnormal apoptosis and development of novel therapeutics for diseases. Here we provide a brief overview of molecular players of programmed cell death with discussion on the role of caspases, modifications, ubiquitylation in apoptosis, removal of the apoptotic body and its relevance to diseases.

The role of the apoptosis-related protein BCL-B in the regulation of mitophagy in hepatic stellate cells during the regression of liver fibrosis

The clearance of activated hepatic stellate cells (HSCs) by apoptosis is critical for the reversibility of hepatic fibrosis. Mitochondrial homeostasis is regulated by mitophagy, which is an efficient way of clearing injured mitochondria that plays an important role in apoptosis. However, the role of mitophagy in apoptosis in HSCs and hepatic fibrosis is still unclear. Here, we show that mitophagy is enhanced in parallel with increased apoptosis in hepatic stellate cells during the reversal of hepatic fibrosis. The inhibition of mitophagy suppressed apoptosis in HSCs and aggravated hepatic fibrosis in mice. In contrast, the activation of mitophagy induced apoptosis in HSCs. Furthermore, we confirmed that BCL-B, which is a member of the BCL-2 family, is a regulator mediating mitophagy-related apoptosis. The knockdown of BCL-B resulted in increased apoptosis and mitophagy in HSCs, while the overexpression of BCL-B caused the opposite effects. BCL-B inhibited the phosphorylation of Parkin (a key regulator of mitophagy) and directly bound phospho-Parkin. Altogether, enhanced mitophagy promotes apoptosis in HSCs during the reversal of hepatic fibrosis. BCL-B suppresses mitophagy in HSCs by binding and suppressing phospho-Parkin, thereby inhibiting apoptosis. BCL-B-dependent mitophagy is a new pathway for the regulation of apoptosis in HSCs during the regression of hepatic fibrosis.

Clearing away defective mitochondria helps destroy cells in the liver that contribute to tissue scarring; the signaling pathway involved offers a new therapeutic target. Hui-Qing Jiang and colleagues from the Hebei Institute of Gastroenterology in Shijiazhuang, China, induced liver fibrosis in mice and showed that as the animals recovered and the damage to their liver tissue was reversed, injured mitochondria were cleared from fibrosis-causing cells in tandem with the cells’ controlled destruction. Experimentally inhibiting the process of mitochondrial clearance also inhibited cell death and aggravated fibrotic scarring in the mice. The researchers identified a signaling pathway that regulates mitochondrial cleanup and, in turn, also controlled cell death. Targeting this pathway offer a potential new therapeutic strategy for reversing liver fibrosis in patients.

Aminoglucose-functionalized, redox-responsive polymer nanomicelles for overcoming chemoresistance in lung cancer cells

Background

Chemotherapeutic drugs used for cancer therapy frequently encounter multiple-drug resistance (MDR). Nanoscale carriers that can target tumors to accumulate and release drugs intracellularly have the greatest potential for overcoming MDR. Glucose transporter-1 (GLUT-1) and glutathione (GSH) overexpression in cancer cells was exploited to assemble aminoglucose (AG)-conjugated, redox-responsive nanomicelles from a single disulfide bond-bridged block polymer of polyethylene glycol and polylactic acid (AG-PEG-SS-PLA). However, whether this dual functional vector can overcome MDR in lung cancer is unknown.

Results

In this experiment, AG-PEG-SS-PLA was synthetized successfully, and paclitaxel (PTX)-loaded AG-PEG-SS-PLA (AG-PEG-SS-PLA/PTX) nanomicelles exhibited excellent physical properties. These nanomicelles show enhanced tumor targeting as well as drug accumulation and retention in MDR cancer cells. Caveolin-dependent endocytosis is mainly responsible for nanomicelle internalization. After internalization, the disulfide bond of AG-PEG-SS-PLA is cleaved in the presence of high intracellular glutathione levels, causing the hydrophobic core to become a polar aqueous solution, which subsequently results in nanomicelle disassembly and the rapid release of encapsulated PTX. Reduced drug resistance was observed in cancer cells in vitro. The caspase-9 and caspase-3 cascade was activated by the AG-PEG-SS-PLA/PTX nanomicelles through upregulation of the pro-apoptotic proteins Bax and Bid and suppression of the anti-apoptotic protein Bcl-2, thereby increasing apoptosis. Furthermore, significantly enhanced tumor growth inhibition was observed in nude mice bearing A549/ADR xenograft tumors after the administration of AG-PEG-SS-PLA/PTX nanomicelles via tail injection.

Conclusions

These promising results indicate that AG-PEG-SS-PLA/PTX nanomicelles could provide the foundation for a paradigm shift in MDR cancer therapy.

Electronic supplementary material

The online version of this article (10.1186/s12951-017-0316-z) contains supplementary material, which is available to authorized users.

The STI and UBA Domains of UBQLN1 Are Critical Determinants of Substrate Interaction and Proteostasis

There are five Ubiquilin proteins (UBQLN1-4, UBQLN-L), which are evolutionarily conserved and structurally similar. UBQLN proteins have three functional domains: N-terminal ubiquitin-like domain (UBL), C-terminal ubiquitin-associated domain (UBA), and STI chaperone-like regions in the middle. Alterations in UBQLN1 gene have been detected in a variety of disorders ranging from Alzheimer's disease to cancer. UBQLN1 has been largely studied in neurodegenerative disorders in the context of protein quality control. Several studies have hypothesized that the UBA domain of UBQLN1 binds to poly-ubiquitin chains of substrate and shuttles it to the proteasome via its UBL domain for degradation. UBQLN1 either facilitates degradation (Ataxin3, EPS15) or stabilizes (PSEN1/2, BCLb) substrates it binds to. The signal that determines this fate is unknown and there is conflicting data to support the existing working model of UBQLN1. Using BCLb as a model substrate, we characterized UBQLN1-substrate interaction. We identified the first two STI domains of UBQLN1 as critical for binding to BCLb. Interaction of UBQLN1 with BCLb is independent of ubiquitination of BCLb, but interaction with ubiquitin via UBA domain is required for stabilization of BCLb. Similarly, we showed that UBQLN1 interacts with IGF1R and ESYT2 through the STI domains and stabilizes these proteins through its UBA domain. Interactions that are not dependent on STI domains, for example, UBL mediated interaction with PSMD4 and BAG6, do not appear to be stabilized by UBQLN1. We conclude that fate of substrates that UBQLN1 associates with, is interaction domain specific. J. Cell. Biochem. 118: 2261-2270, 2017. © 2017 Wiley Periodicals, Inc.

BCL-B (BCL2L10) is overexpressed in patients suffering from multiple myeloma (MM) and drives an MM-like disease in transgenic mice

Luciano et al. generate transgenic mice expressing the Bcl-B gene under the control of the VH promoter and Eµ enhancer and show that these mice recapitulate the characteristic features of human MM.

Multiple myeloma (MM) evolves from a premalignant condition known as monoclonal gammopathy of undetermined significance (MGUS). However, the factors underlying the malignant transformation of plasmocytes in MM are not fully characterized. We report here that Eµ-directed expression of the antiapoptotic Bcl-B protein in mice drives an MM phenotype that reproduces accurately the human disease. Indeed, with age, Eµ-bcl-b transgenic mice develop the characteristic features of human MM, including bone malignant plasma cell infiltration, a monoclonal immunoglobulin peak, immunoglobulin deposit in renal tubules, and highly characteristic bone lytic lesions. In addition, the tumors are serially transplantable in irradiated wild-type mice, underlying the tumoral origin of the disease. Eµ-bcl-b plasmocytes show increased expression of a panel of genes known to be dysregulated in human MM pathogenesis. Treatment of Eµ-bcl-b mice with drugs currently used to treat patients such as melphalan and VELCADE efficiently kills malignant plasmocytes in vivo. Finally, we find that Bcl-B is overexpressed in plasmocytes from MM patients but neither in MGUS patients nor in healthy individuals, suggesting that Bcl-B may drive MM. These findings suggest that Bcl-B could be an important factor in MM disease and pinpoint Eµ-bcl-b mice as a pertinent model to validate new therapies in MM.

Emerging understanding of Bcl-2 biology: Implications for neoplastic progression and treatment

Bcl-2, the founding member of a family of apoptotic regulators, was initially identified as the protein product of a gene that is translocated and overexpressed in greater than 85% of follicular lymphomas (FLs). Thirty years later we now understand that anti-apoptotic Bcl-2 family members modulate the intrinsic apoptotic pathway by binding and neutralizing the mitochondrial permeabilizers Bax and Bak as well as a variety of pro-apoptotic proteins, including the cellular stress sensors Bim, Bid, Puma, Bad, Bmf and Noxa. Despite extensive investigation of all of these proteins, important questions remain. For example, how Bax and Bak breach the outer mitochondrial membrane remains poorly understood. Likewise, how the functions of anti-apoptotic Bcl-2 family members such as eponymous Bcl-2 are affected by phosphorylation or cancer-associated mutations has been incompletely defined. Finally, whether Bcl-2 family members can be successfully targeted for therapeutic advantage is only now being investigated in the clinic. Here we review recent advances in understanding Bcl-2 family biology and biochemistry that begin to address these questions.

Regulation of the DNA damage response by ubiquitin conjugation

In response to DNA damage, cells activate a highly conserved and complex kinase-based signaling network, commonly referred to as the DNA damage response (DDR), to safeguard genomic integrity. The DDR consists of a set of tightly regulated events, including detection of DNA damage, accumulation of DNA repair factors at the site of damage, and finally physical repair of the lesion. Upon overwhelming damage the DDR provokes detrimental cellular actions by involving the apoptotic machinery and inducing a coordinated demise of the damaged cells (DNA damage-induced apoptosis, DDIA). These diverse actions involve transcriptional activation of several genes that govern the DDR. Moreover, recent observations highlighted the role of ubiquitylation in orchestrating the DDR, providing a dynamic cellular regulatory circuit helping to guarantee genomic stability and cellular homeostasis (Popovic et al., 2014). One of the hallmarks of human cancer is genomic instability (Hanahan and Weinberg, 2011). Not surprisingly, deregulation of the DDR can lead to human diseases, including cancer, and can induce resistance to genotoxic anti-cancer therapy (Lord and Ashworth, 2012). Here, we summarize the role of ubiquitin-signaling in the DDR with special emphasis on its role in cancer and highlight the therapeutic value of the ubiquitin-conjugation machinery as a target in anti-cancer treatment strategy.

How chemistry supports cell biology: the chemical toolbox at your service

Chemical biology is a young and rapidly developing scientific field. In this field, chemistry is inspired by biology to create various tools to monitor and modulate biochemical and cell biological processes. Chemical contributions such as small-molecule inhibitors and activity-based probes (ABPs) can provide new and unique insights into previously unexplored cellular processes. This review provides an overview of recent breakthroughs in chemical biology that are likely to have a significant impact on cell biology. We also discuss the application of several chemical tools in cell biology research.

Antiapoptotic potency of Bcl-2 proteins primarily relies on their stability, not binding selectivity

All 6 human prosurvival Bcl-2 proteins can drive cancer development and contribute to therapy resistance. However, their relative abilities to protect cells against cancer therapy were not examined previously. We report that Bcl-2, Bcl-xL, or Bcl-w consistently protected leukemic cells better than Bcl-B, Bfl-1, or Mcl-1 against a wide variety of anticancer regimens. Current thinking would attribute this to differences in their ability to bind to BH3-only proteins, Bax, and Bak. To address this, we established the first complete, quantitative cellular interaction profile of all human prosurvival Bcl-2 proteins with all their proapoptotic relatives. Binding was unexpectedly promiscuous, except for Bad and Noxa, and did not explain the differential antiapoptotic capacity of the Bcl-2 proteins. Rather, Bcl-B, Bfl-1, or Mcl-1 proved less potent due to steady-state or drug-induced proteasomal degradation. All 6 Bcl-2 proteins similarly protected against the diverse anticancer regimens when expressed at equal protein levels, in agreement with their broad interaction profile. Therefore, clinical diagnostics should include all family members and should be performed at the protein rather than at the messenger RNA level. In drug development, targeting the ubiquitination machinery of prosurvival Bcl-2 proteins will complement and potentially improve on targeting Bcl-2 protein interactions with BH3 mimetics.

Structural biology of the Bcl-2 family and its mimicry by viral proteins

Intrinsic apoptosis in mammals is regulated by protein–protein interactions among the B-cell lymphoma-2 (Bcl-2) family. The sequences, structures and binding specificity between pro-survival Bcl-2 proteins and their pro-apoptotic Bcl-2 homology 3 motif only (BH3-only) protein antagonists are now well understood. In contrast, our understanding of the mode of action of Bax and Bak, the two necessary proteins for apoptosis is incomplete. Bax and Bak are isostructural with pro-survival Bcl-2 proteins and also interact with BH3-only proteins, albeit weakly. Two sites have been identified; the in-groove interaction analogous to the pro-survival BH3-only interaction and a site on the opposite molecular face. Interaction of Bax or Bak with activator BH3-only proteins and mitochondrial membranes triggers a series of ill-defined conformational changes initiating their oligomerization and mitochondrial outer membrane permeabilization. Many actions of the mammalian pro-survival Bcl-2 family are mimicked by viruses. By expressing proteins mimicking mammalian pro-survival Bcl-2 family proteins, viruses neutralize death-inducing members of the Bcl-2 family and evade host cell apoptosis during replication. Remarkably, structural elements are preserved in viral Bcl-2 proteins even though there is in many cases little discernible sequence conservation with their mammalian counterparts. Some viral Bcl-2 proteins are dimeric, but they have distinct structures to those observed for mammalian Bcl-2 proteins. Furthermore, viral Bcl-2 proteins modulate innate immune responses regulated by NF-κB through an interface separate from the canonical BH3-binding groove. Our increasing structural understanding of the viral Bcl-2 proteins is leading to new insights in the cellular Bcl-2 network by exploring potential alternate functional modes in the cellular context. We compare the cellular and viral Bcl-2 proteins and discuss how alterations in their structure, sequence and binding specificity lead to differences in behavior, and together with the intrinsic structural plasticity in the Bcl-2 fold enable exquisite control over critical cellular signaling pathways.

New perspective on the stabilization and degradation of the F-box protein COI1 in Arabidopsis

The F-box protein CORONATINE INSENSITIVE1 (COI1) assembles into SCF(COI1) complexes and recruits its substrate JAZ proteins for ubiquitination and degradation to regulate diverse aspects of jasmonate-regulated plant developmental processes and defense responses. However, the dynamically regulation of COI1 protein abundance in plants remains unknown. In our Plant Cell paper, through genetic, biochemical analysis and in vitro degradation assays, we demonstrated that the COI1 protein is strictly regulated by a dynamic balance of SCF(COI1)-mediated stabilization and 26S proteasome-mediated degradation, and maintained at a proper level suitable for essential biological processes in plants. In this addendum, we provided additional insights and speculation on the stabilization and degradation of COI1.